Insulin is a miraculous peptide hormone. It demonstrates unparalleled ability to lower glucose in virtually all forms of diabetes. Unfortunately, its pharmacology is not glucose sensitive and as such it is capable of excessive action that can lead to life-threatening hypoglycemia. Inconsistent pharmacology is a hallmark of insulin therapy such that it is extremely difficult to normalize blood glucose without occurrence of hypoglycemia. Furthermore, native insulin is of short duration of action and requires modification to render it suitable for use in control of basal glucose. One central goal in insulin therapy is designing an insulin formulation capable of providing a once a day time action. Extending the action time of an insulin dosage can be achieved by decreasing the solubility of insulin at the site of injection.
There are three proven and distinct molecular approaches to reducing solubility and they include; (1) formulation of insulin as an insoluble suspension with zinc, (2) increase in its isoelectric point to physiological pH through addition of cationic amino acids, (3) covalent modification to provide a hydrophobic ligand that reduces solubility and binds albumin. All of these approaches are limited by the inherent variability that occurs with precipitation at the site of injection, and with subsequent re-solubilization & transport to blood as an active hormone.
Prodrug chemistry offers an alternative mechanism to precisely control the onset and duration of insulin action after clearance from the site of administration and equilibration in the plasma at a highly defined concentration. The central virtue of such an approach relative to current long-acting insulin analogs and formulations is that the insulin reservoir is not the subcutaneous fatty tissue where injection occurs, but rather the blood compartment. This removes the variability in precipitation and solubilization. It also enables administration of the peptide hormone by routes other than a subcutaneous injection. To build a successful prodrug-hormone, an active site structural address is needed that can form the basis for the reversible attachment of a prodrug structural element. The structural address needs to offer two key features; (1) the potential for selective chemical modification and (2) the ability to provide full activity in the native form upon removal of the prodrug structural element.
Insulin is a two chain heterodimer that is biosynthetically derived from a low potency single chain proinsulin precursor through enzymatic processing. Human insulin is comprised of two peptide chains (an “A chain” (SEQ ID NO: 1) and “B chain” (SEQ ID NO: 2)) bound together by disulfide bonds and having a total of 51 amino acids. The C-terminal region of the B-chain and the two terminal ends of the A-chain associate in a three-dimensional structure to assemble a site for high affinity binding to the insulin receptor. The native insulin structure has limited unique chemical elements at the active site residues that might be used for selective assemble of an amide linked prodrug element. Two sites that could be modified to provide elements for the attachment of a prodrug element include the tyrosine residue at position 19 of the native A chain (the “A19 tyrosine”) and the phenylalanine residue at position 24 of the native B chain (the “B24 phenylalanine”). Both of these two amino acids are of central importance in insulin action. However, these two amino acids have also proven highly restrictive in the type of structural change that can be introduced and still maintain full potency.
As disclosed herein applicants have discovered full potency insulin analogs that have been modified at position A19 and could potentially be used to assemble an insulin prodrug derivative.